Click the picture
to see the filament image centred.

Click for Micrographia Home Page. Centreing the Microscope Lamp (2).

Filament image is displaced too far to the right.

Use the lamp centreing screws to quickly move the image about. It becomes very clear where the central position is, and accurate centration is possible even when the image is not sharp.  Focus back and forth to confirm that artefacts are concentric.